1. Field of the Invention
The invention pertains to apparatus and methods for chemically depositing a solid state cathode film for alkali ion batteries.
2. Description of Related Art
Lithium (Li) ion, sodium (Na) ion, or Li—Na multi-ion secondary batteries are known to be high energy density batteries. For sufficient power, thick cathodes are employed in these batteries. Over the years these cathodes were fabricated by a series of complex and expensive techniques. These include forming nanoscale powders of active cathode material, mixing the active powder with an inert organic binder dissolved in appropriate solvent to form a slurry, using various slurry coating techniques to form the thick film of the cathode on a metallic substrate, followed by calendaring and drying processes to fully stabilize and form the cathode. Another cathode formation scheme involves mixing the cathode material (as a nano-particle powder) with the binder powder and pressing the mixture to form a pellet or a plate cathode, followed by drying. The inert binder content in these cathodes could be as high as 30% and unnecessarily lowers the power density of batteries containing them.
Several methods for making nanoscale powders of cathode materials have been disclosed:
In U.S. Pat. No. 6,350,543 or U.S. Pat. No. 7,258,821 LixMnyNizCouAlwOn cathode powder, where 0<x≦2, the sum of y+z+u+w is about 1 to 2, and 2≦n≦4, and 0.7≦y/(y+z+u+w)<1.0 was made according to the followings steps: (a) making a solution of manganese nitrate, nickel nitrate, cobalt nitrate, and aluminum nitrate in the appropriate volumetric ratio in water; (b) adding LiOH solution to the above quaternary nitrate mixture solution to effect homogenous co-precipitation of the respective metal hydroxides; washing the precipitate to eliminate lithium nitrates; (c) slowly drying the resulting paste and oxidizing it at about 80° C. for 1 to 5 days; (d) mixing the dried oxidized precipitate with sufficient amount of LiOH, and calcining the powder at about 750° C. for about 24 hours in air to form LixMnyNizCouAlwOn cathode powder.
In U.S. Pat. No. 7,008,608 Li[CoxLi(1/3-x/3)Mn(2/3-2x/3)]O2 (0.05<x<0.9) cathode powder was synthesized using stoichiometric amount of lithium acetate dihydrate, lithium nitrate, manganese acetate tetrahydrate, cobalt nitrate hexahydrate dissolved in distilled water. This solution was heated at about 150° C. to form a gel. The gel was then calcined at 400° C. to remove the organic content and remaining moisture to form lumps of metal oxide. The lumps were ground into fine particles and calcined at 500° C. for about 3 hours. The resulting powder was then sintered at 1000° C. for 6 hours followed by additional grinding to form the desired cathode metal oxide powder.
In U.S. Pat. No. 5,783,333 or U.S. Pat. No. 6,379,842 lithium nickel cobalt metal oxide is described, with general formula LixNiyCozMO2, where M is selected from the group consisting of aluminum, titanium, tungsten, chromium, molybdenum, magnesium, tantalum, silicon, and combination thereof, x is between about 0 and 1, the sum of x+z+n is about 1, n ranges between 0 to about 0.25, y and z are both greater than 0, and the ratio z/y ranges from above 0 to about 1/3. The patent also disclosed LixMn2-rM1rO4, where r is a value between 0 and 1, M1 is chromium, titanium, tungsten, nickel, cobalt, iron, tin, zinc, zirconium, silicon. LixNiyCozMO2 is formed by combining specified lithium containing compound with a specified cobalt containing compound, a specified nickel containing compound, and the specified metal (M) containing compound. The combined various components are well mixed and then thermally reacted at a temperature between about 400 and 1300° C. in oxygen ambient. The compounds of choice are the nitrate, hydroxide, acetate, or carbonate of Li, Co, Ni and M. LixMn2-rM1rO4 is also similarly processed, but the source of Mn is preferably MnO2 and the oxide of M1 or the pure metal nanoparticle of M1.
U.S. Pat. No. 5,718,989 or U.S. Pat. No. 5,795,558 describe LiNi1-x-y-zCoxMnyAlzO2 wherein x, y, and z satisfy relations of 0≦y≦0.3, 0≦x≦0.25, 0<z≦0.15. The co-precipitated Ni1-x-y-zCoxMny(OH)2 was mixed with Al(OH)3 and LiOH in the predetermined proportion, the mixture was calcined at 720° C. for 40 hours under an atmosphere of oxygen partial pressure of 0.5 atm. After calcining, these were ground to an average of 3.5 μm by ball mill to obtain LiNi1-x-y-zCoxMnyAlzO2 cathode powder.
U.S. Pat. No. 5,589,300 discloses the generation of a more homogeneous electrode material powder consisting of particles of controlled sizes. The particles are generated by precipitation or homogeneous reaction. For example for the formation LiMn2O4 powder: MnNO3 and LiNO3 are dissolved in ethyl alcohol, alcohol, or water. The precursor is nebulized into aerosol or small droplets. The droplets are swept into a hot tube where solvent is driven away and the reagents in each droplet react to form LiMn2O4 particles. The particles are then collected electrostatically on a metallic collector. These particles may be further heat treated before use. The cathode film is then formed by mixing the powder with suitable organic binder to form a slurry which is applied to a metal current collector to form the film.
It is therefore clear that traditional methods of making the powder and deploying the powder to make the film are cumbersome, and more streamlined methods are needed to enable wider adoption of lithium ion batteries.
Vacuum deposition techniques, sputtering, chemical vapor deposition, Pulse Laser deposition, have been adopted to grow organic binder free inorganic cathode films. These processes are slow and expensive, and the grown films are thin, less than 5 μm. The latter are therefore suitable only for microbatteries. Adopting these processes to grow thicker film on a large area would not be economical, because the capital equipment cost or/and operation cost will be too high [see, for example, Journal of the Electrochemical Society, 147(2):517-23 (2000); Materials Research Bulletin 43:1913-42 (2008)].
U.S. Pat. No. 6,582,481 describes a method of producing a layer of lithium metal oxide film comprising the steps of: (a) providing a solution having a mixture of lithium (2,2,6,6-tetramethyl-3,5-heptadionate) and cobalt (III) acetyl acetanoate dissolved in an organic solvent consisting diglyme, toluene and (2,2,6,6-tetramethyl-3,5-heptadionate); (b) processing the solution to form a mist using an ultrasonicator; (c) heating the solution mist to a vapor state at around 200° C.; and (d) depositing the vapor upon a substrate heated at 400° C. Although this is a non vacuum process, the use of organometallic precursors and organic solvents will not make this process a cheap one. In addition, ultrasonic generation of the mist will make the process a slow process, hence, suitable only for micobatteries.
Similarly, Kim et al., in Chemical Vapor Deposition 9(4):187-92 (2003), dissolved LiNO3 and Mn(NO3)2 in 2-methoxyethanol and ethylene glycol mixed solvent. The solution was transformed into ultrafine mist by 1.65 MHz ultrasonic nebulizer. The mist was carried to the substrate by argon gas for film deposition at room temperature. The film was baked at 230° C. then 400° C. and recrystallized at about 700° C. for binder free LiMnO4 cathode film formation. The process is suitable for microbatteries as the reported deposition rate was about 100 Å/minute.
Binder free cathode films have also been grown by electrostatic spray deposition. Here, the solution consisting of lithium salt and metal salt dissolved in ethanol or ethanol and butyl carbitol mixture is pumped to a metallic capillary nozzle. A DC voltage above 5 kV applied between the metallic nozzle and the heated metallic substrate generates a mist by electrohydrodynamic force. The electrostatic force then moves the mist to the hot substrate at temperature between 240 to 450° C. where the film gets deposited by pyrolysis of the mist. About 1 to 5 μm thick film could be deposited by this technique per hour, therefore very suitable for microbatteries. [See, for example, C. H. Chen et al.; Solid State Ionics; 86-88:1301-06 (1996).]